Subject: Re: Negotiating MRU

From: [email-masked]
Date: Tue, 12 Sep 2000 12:39:12 -0400

>>>[email-masked] writes:
>>>If Device-A negotiates an MRU of 1024 and Device-B negotiates an MRU of 2048,
>>>what happens to
>>>packets sent from Device-B to Device-A that are larger than 1024 (assuming
>>>non-static LSPs)?

>[email-masked] writes:
>I assume by the references to LERs and LSPs that this is actually an
>MPLS question, not a PPP question.

I don't think so. What I was wondering is that at the time PPP is negotiated,
you don't know who
you are going to be sending packets to. So when you negotiate the link MRU, it
is independent
of any other device that may have also negotiated PPP.

In the example I gave, my question has to do with: If Device-A and Device-B
independently negotiate
PPP and then end up talking to each other later on, what if one sends a packet
larger than what the
other can receive?

Thanks,
MK

(requires fixed font to make sense of below graphic)

|========|  MRU=1024  |=|   |~~~~~~~~~~~|   |=|  MRU=2048  |========|
| -A-    |----------->|L|-->|           |-->|L|----------->| -B-    |
| PoS    |            |E|   | N/W Cloud |   |E|            | PoS    |
| Device |<-----------|R|<--|           |<--|R|<-----------| Device |
|========|  MRU=1024  |=|   |~~~~~~~~~~~|   |=|  MRU=2048  |========|

---

From: James Carlson <[email-masked]>
Date: Tue, 12 Sep 2000 12:52:03 -0400 (EDT)

[email-masked] writes:
> I don't think so. What I was wondering is that at the time PPP is
> negotiated, you don't know who you are going to be sending packets
> to. So when you negotiate the link MRU, it is independent of any
> other device that may have also negotiated PPP.

Sure.

> PPP and then end up talking to each other later on, what if one sends a packet

Assuming all of the paths within the cloud are >= 2048, then it's the
LER on the last hop that will discard the packet.  If that assumption
isn't true, then one of the LSRs in the middle will do it.  The end
points are just part of the concern; every hop in that cloud is a
potential problem.  (Consider what happens if one of the links in the
middle of the network has an MTU set to 512.  Neither device has this
value, and yet it's a constraint of the LSPs going through that link.)

This is an error case; large packets like this shouldn't be sent by a
properly implemented MPLS-speaking device.  In MPLS, signaling is used
to establish the path MTU.

The same is true for network-layer protocols, such as IP.  With IPv4,
fragmentation within IP is sometimes possible.  With IPv6, it's not.

It's not the link layer's problem to deal with network level issues.
This isn't a PPP issue.

-- 
James Carlson                                  <[email-masked]>
"PPP Design and Debugging" --- http://people.ne.mediaone.net/carlson/ppp

---

From: Farrell Woods <[email-masked]>
Date: Tue, 12 Sep 2000 13:13:00 -0400

> I don't think so. What I was wondering is that at the time PPP [sic] is negotiated,
> you don't know who you are going to be sending packets to.

Right.  That's why this isn't a PPP issue.  Whatever you use for a network
layer will have to deal with it.  If this were IP then then the "point B"
device would get back a ICMP message, probably from some device in
the "N/W cloud" indicating that fragmentation is needed.  It's then
incumbent on the "point B" device to do that fragmenting.  If things
are working right then the device that can't handle the larger message
size will also send back a hint as to what the largest message it can
handle is.  But then there's no guarantee that something else in the
cloud might not impose a smaller message size.

The issue is independent of PPP: what if I have a FDDI ring (where I can
have >4K packets) bridged to an Ethernet (1K packets.)  Each link needs
to let its network layer know somehow what the largest message size is.
But it's the network layer that needs to do the fragmentation and
reassembly.

	-- Farrell